1. Field of the Invention
This invention relates to a steering device for a vehicle, and more particularly to a steering device in which the steering ratio, the ratio of the rotating angle of the steering wheel to the angle by which the dirigible wheels are turned as a result of the rotation of the steering wheel by the rotating angle, is variable in response to the rotating speed of the steering wheel.
2. Description of the Prior Art
Generally, the dirigible wheels of a vehicle are operatively connected with the steering wheel by way of a steering linkage to be turned in response to rotation of the steering wheel. The steering angle of the dirigible wheels, i.e., the angle by which the dirigible wheels are turned, solely depends upon the rotating angle of the steering wheel and is independent of the rotating speed of the steering wheel.
In case of emergency (for example when clearing an obstruction in the road), it is preferred that the steering ratio be small so that the dirigible wheels can be turned through a large angle by rotation of the steering wheel through a small angle thereby obtaining a quick response of the dirigible wheels to the steering operation. In order to meet such a requirement, there has been proposed in Japanese Patent Publication 53(1978)-10731 a steering device in which the steering ratio is variable in response to the rotating speed of the steering wheel so that the steering ratio is decreased when the steering wheel is rotated at a high speed or quickly as in the case of emergency. This steering device utilizes viscosity or an electromagnetic coupling to make the steering ratio variable in response to the rotating speed of the steering wheel.
However, the variable steering ratio of the steering device proposed in the Japanese Patent Publication is independent of the speed of the vehicle and the steering torque of the steering wheel. Therefore, if the steering wheel is operated when the vehicle speed is high in such a manner as when the vehicle speed is low, the vehicle will make a dangerous sharp turn. Further, when the steering wheel is rotated with a large torque as is the case when the steering wheel is operated with the vehicle being at rest, a damping effect responsive to the steering torque cannot be obtained to give a bad feeling to the driver in the steering operation.